my quantum always gets very warm when streaming music or using the gps.
the quantum is completely aluminum, it absorps heat from the cpu and cell and gps chips and all of those chips are really close together. This is normal for it to get alot of surface heat. Windows phone has a build-in temp sensor that will turn the phone off if it gets too hot. don't worry about it if that overheating screen isn't showing up.
one of the reasons this happens is the OS is either running at 200mhz at idle and 1Ghz when running an app. there is no throttle.
I hope this help.